examples: move the particle module in modules/particle

The goal is enabling running `v build-examples` from outside vroot.
The modules/ folders are already skipped when building examples.

examples/sokol/particles/modules/particle/vec2/vec2.v

@@ -0,0 +1,89 @@
+// Copyright(C) 2019 Lars Pontoppidan. All rights reserved.
+// Use of this source code is governed by an MIT license file distributed with this software package
+module vec2
+
+pub struct Vec2 {
+pub mut:
+	x f64
+	y f64
+}
+
+pub fn (mut v Vec2) zero() {
+	v.x = 0.0
+	v.y = 0.0
+}
+
+pub fn (mut v Vec2) from(src Vec2) {
+	v.x = src.x
+	v.y = src.y
+}
+
+// * Addition
+// + operator overload. Adds two vectors
+pub fn (v1 Vec2) +(v2 Vec2) Vec2 {
+	return Vec2{v1.x + v2.x, v1.y + v2.y}
+}
+
+pub fn (v Vec2) add(vector Vec2) Vec2 {
+	return Vec2{v.x + vector.x, v.y + vector.y}
+}
+
+pub fn (v Vec2) add_f64(scalar f64) Vec2 {
+	return Vec2{v.x + scalar, v.y + scalar}
+}
+
+pub fn (mut v Vec2) plus(vector Vec2) {
+	v.x += vector.x
+	v.y += vector.y
+}
+
+pub fn (mut v Vec2) plus_f64(scalar f64) {
+	v.x += scalar
+	v.y += scalar
+}
+
+// * Subtraction
+pub fn (v1 Vec2) -(v2 Vec2) Vec2 {
+	return Vec2{v1.x - v2.x, v1.y - v2.y}
+}
+
+pub fn (v Vec2) sub(vector Vec2) Vec2 {
+	return Vec2{v.x - vector.x, v.y - vector.y}
+}
+
+pub fn (v Vec2) sub_f64(scalar f64) Vec2 {
+	return Vec2{v.x - scalar, v.y - scalar}
+}
+
+pub fn (mut v Vec2) subtract(vector Vec2) {
+	v.x -= vector.x
+	v.y -= vector.y
+}
+
+pub fn (mut v Vec2) subtract_f64(scalar f64) {
+	v.x -= scalar
+	v.y -= scalar
+}
+
+// * Multiplication
+pub fn (v1 Vec2) *(v2 Vec2) Vec2 {
+	return Vec2{v1.x * v2.x, v1.y * v2.y}
+}
+
+pub fn (v Vec2) mul(vector Vec2) Vec2 {
+	return Vec2{v.x * vector.x, v.y * vector.y}
+}
+
+pub fn (v Vec2) mul_f64(scalar f64) Vec2 {
+	return Vec2{v.x * scalar, v.y * scalar}
+}
+
+pub fn (mut v Vec2) multiply(vector Vec2) {
+	v.x *= vector.x
+	v.y *= vector.y
+}
+
+pub fn (mut v Vec2) multiply_f64(scalar f64) {
+	v.x *= scalar
+	v.y *= scalar
+}